(1) Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to a technique for changing an image forming condition of an image forming apparatus which forms an image using an electrophotographic method.
(2) Description of the Related Art
In recent years, so called "tandem-type" full-color image forming apparatuses have been increasingly used. In a tandem-type full-color image forming apparatus, image forming units including photosensitive drums as main components are set along a transport belt, and toner images for different colors formed on the image forming units are transferred onto a transfer material, such as a recording sheet, with the transferred images being superimposed.
Using this tandem-type image forming apparatus, a full-color image is obtained after the recording sheet passes by the photosensitive drums only once, thereby improving a copying operation speed. However, when only one image forming unit is used for forming a black image (referred to as the "monochrome image" hereinafter), the recording sheet still comes into contact with the other three image forming units during transportation. For this reason, the three image forming units which are not used for the image formation still need to be rotated. This results in needless wear and tear on the surfaces of the three photosensitive drums and cleaning members that are provided for the photosensitive drums, shortening the lifespans of these components. Also, toner in developing units is unnecessarily consumed.
To address this problem, Japanese Laid-Open Patent Applications No. 6-258914 discloses a tandem-type image forming apparatus which has a transport belt for transporting a recording sheet come in contact with all of photosensitive drums when forming a full-color image (referred to as the "color copy mode" hereinafter), and has the transport belt move downward using a moving mechanism to separate the transport belt from the photosensitive drums that are not used when forming a black image using only the image forming unit for black (referred to as the "monochrome copy mode" hereinafter). Here, rotations of the photosensitive drums separated from the transport belt are stopped, thereby preventing needless wear and tear on the photosensitive drums.
However, when using the image forming apparatus having such a moving mechanism that separates the photosensitive drums and the transport belt, tension of the transport belt may fluctuate and the transport belt may slide along a driving roller due to the moving operations of the moving mechanism. Also, it is also difficult for the moving mechanism to precisely position the transport belt at the uppermost and lowermost positions. If the transport belt is not stopped at the correct uppermost and lowermost positions, transfer positions of the photosensitive drums are inconsistent. As a result, every time the transport belt is shifted by the moving mechanism, transfer positions of the photosensitive drums are slightly changed, thereby causing color displacements on a transferred color image.
This problem is described for an image forming apparatus that switches the copy mode between the monochrome copy mode and the color copy mode. However, when forming an image selectively using a plurality of image forming means, like photosensitive drums, problems, such as the aforementioned color displacements, result in lower quality for the reproduced image.